The present invention relates to a lithographic process for the production of structures for use in micro-devices and nano-devices, in particular in the fields of micro-electronics, nano-electronics, micro-fluidics, optics, opto-electronics, magnetic memories, micro-mechanics, nano-mechanics and sensors.
One of the reasons for the rapid and continuous progress which has been possible in the field of micro-electronics during the last three decades is the reduction in the dimensions of devices and their massive integration into single chips. This reduction in scale of the dimension of devices seems also to be influencing other fields such as optics, mechanics and memorisation of data with the same degree of intensity and has given rise to new fields of technology such as micro-fluidics, micro-mechanics and nano-mechanics.
The fabrication of such a variety of devices has posed challenges in terms of productive capacity, resolution, accuracy, flexibility, reliability and cost, and has given an impulse to research into new lithographic techniques.
In this context one emerging technology is represented by nano-impression lithography (“Nano Imprint Lithography” or “NIL”) the invention of which has opened a new route in the field of lithography which makes it possible to make less use of rays of energetic particles such as photons, electrons or ions, for the purpose of printing a certain pattern in relief on a thin polymeric film.
The general principle on which NIL is based is that of replicating a pattern in relief present on the surface of a die by pressing this latter onto a film of material which can be deformed under pressure deposited on a substrate. Thus, this material tends to fill the cavities of the die and to conform to its profile. The die is then removed leaving its profile impressed into the polymeric film which is then further treated in a manner well-known to those skilled in the art by means of attack with reactive ions, ion implantation or “lift-off metal”. The conventional procedures of NIL have a resolution up to 10 nm with low equipment costs as well as the potential to be used in the above-mentioned technical sectors, including micro-electronics.
The conventional procedures of NIL involve bringing a die having a profile in micro-structured or nano-structured relief on one of its surfaces into contact with a substrate covered in a thin film of a thermoplastic material, introducing the assembly of die and covered substrate between the plates of a press, heating these latter and holding the assembly pressed together for a sufficient time for the pattern formed on the surface of the die to be imprinted onto the thermoplastic film. The role of temperature is to soften the thermoplastic material so as to permit it to flow and to reproduce the profile of the die. In general, the thermoplastic material becomes fluid above a temperature known as the glass transition temperature. Above this temperature the viscosity of the thermoplastic material decreases with an increase in temperature. In conventional NIL procedures, after pressing the die against the substrate covered in polymeric material the temperature of the heated plates of the press must be again reduced below the glass transition temperature of the thermoplastic material before releasing the pressure exerted by the plates, in such a way that the pattern in relief impressed onto the polymeric film maintains its form.
Typically, the thermal excursion of one impression cycle is of the order of 100° C. or more so as to guarantee a sufficient variation in the viscosity of the polymeric material. However, the performance of such a prior art cycle involves a series of disadvantages.
In the first place, the die and the substrate covered with polymeric film are subject to a large thermal expansion which makes their accurate positioning problematic.
This makes it difficult to develop processes which necessitate several lithographic stages involving alignment of micro and nano structures with pre-existing structures.
In the second place, the large thermal capacity of the masses involved in the heating/cooling cycle essentially determines the duration of the process, which typically is of the order of several minutes. This time is very much longer than that which is effectively required at standard pressures for the impression of patterns in relief onto thermoplastic film, which is of the order of a few seconds or less.
In the third place, at each thermal cycle the thermal energy stored in the entire system is wasted, with an increase in the energy consumption of the procedure, which becomes more serious the greater the production volumes.
In the fourth place, it is not possible to repeat the printing procedure in different regions of the substrate covered in thermoplastic material in that this latter would melt completely over the area of the substrate each time, causing the disappearance of the previously impressed patterns.
In the fifth place, it is not possible with the known NIL technology to form a pattern in relief on the surface of three dimensional thermoplastic objects in that this would cause a softening of the material throughout the volume of the object with a loss of its overall shape.